Time-of-flight mass spectrometry (TOFMS) is an inexpensive analytical technique that has become the preferred choice for monitoring a variety of chemical separation processes that require mass discrimination. In one TOFMS technique that circumvents the problems associated with pulsing continuous ion beams, called Hadamard transform time-of-flight mass spectrometry (HT-TOFMS), ion sources are continuously monitored with an intrinsic duty cycle of 50% over any mass range. This technique is described in U.S. Pat. No. 6,300,626. A similar technique where the ion beam is deflected in both the “on” and “off” states is described in U.S. Pat. No. 6,870,157. With only a few exceptions, the instrumental set-up is identical to conventional TOFMS instruments. Because of its inherent high duty cycle, uncompromised choice of mass range, and absence of time-varying high voltages (low voltage pulsing), HT-TOFMS holds the promise of being miniaturized into a rugged instrument capable of field operation.
In a HT-TOFMS experiment, ions 12 entering the spectrometer are focused onto a Bradbury-Nielsen gate (BNG) 14, consisting of a two interspersed wire sets as shown in FIG. 1a. As shown in FIG. 1a, in the “on” state, the ions are not deflected, and are directed to inner detector area(s) on the MCP detector after reflection by the reflectron. In the “off” state, the ions are deflected optionally to the outer area(s) on the MCP detector (the amount of deflection being exaggerated in FIG. 1a). FIGS. 1b and 1c describe the principle under which the ion gate operates: the gate's deflection voltage is rapidly modulated ‘on’ and ‘off’ with a known sequence, creating small packets of ions. Principle of operation of a Bradbury-Nielsen ion gate is illustrated in FIG. 1b when the two interleaved wire sets are at the same voltage, ions fly unaffected through the gate, and FIG. 1c when a bias voltage is applied to the two wire sets, ions are deflected off the propagation axis. Multiple ion packets (constituting 50% of the total number of ions) enter the flight chamber during a single scan and interpenetrate one another as they fly toward the detector. The acquired signal, which is the superposition of all the packets' spectra, is deconvoluted using knowledge of the applied modulation sequence. The recovered signal is equivalent to conventional time-of-flight (TOF) spectrum, but now has a dramatically improved signal-to-noise ratio (SNR) owing to the multiplexing scheme. Because of the inherent high duty cycle and the rate of the modulation, HT-TOFMS usually offers scan speeds that are higher than any other form of TOFMS. For a more detailed description of the technique, please see U.S. Pat. No. 6,300,626. Because of these advantages, it is expected that HT-TOFMS will soon be a commonly used technique.
A Bradbury-Nielson gate shutters the ion beam “on” and “off” by applying a 10 MHz sequence of square voltage pulses to the wire set.; The integrity of the multiplexing scheme depends on the discrete, accurate modulation of the beam; the performance of the gate is thus crucial. Bradbury-Nielson gates are used in a multitude of MS and ion mobility analyzers, but few of these applications have the temporal requirements of HT-TOFMS.
Because the deflection efficiency and temporal resolution of these gates depends on the spacing between wires, extensive efforts have been made to develop new methods for producing gates with finely spaced wires. The quality of recorded mass spectra is limited by the ability of the BNG to control the trajectory of the ion beam. Therefore, efforts are underway to optimize the dimensions, ruggedness, and deflection efficiency of the BNG. In particular, a beam modulation device (“BMD”) is desired with fine inter-electrode spacing that can be manufactured in high volume. In order to improve mass resolution and modulation pulse profiles, much effort has been made to produce Bradbury-Nielson gates with minimal spacing between wires. A detailed description of the use of this device in time-of-flight mass spectrometry appeared in 1995 by Viasak et al. See “An interleaved comb ion deflection gate for m/z selection in time-of-flight mass spectrometry,” by P. R. Vlasak et al., Rev. Sci Instrum., 1996, 67, 68–72. In this work, a wire spacing of 1 mm was achieved by weaving a wire through holes on two separate frames and applying tension with a bracing screw between the two frames. A significant reduction of the wire spacing to 0.5 mm was reported in 1998 by Stoermer et al. who used the grooves on two nylon threads to control the wire spacing. This group used two sequential grids to minimize pulse widths. Still, they concluded that further reduction in wire spacing would improve m/z selectivity in TOF experiments.
The next advance in the reduction of the wire spacings was reported by Brock, Rodriguez, and Zare, who were able to construct Bradbury-Nielson gates for their HTM-TOF mass spectrometer with a wire spacing of 0.16 mm, working by hand under a microscope to set the wires in a frame made from a piece of printed circuit board (PCB) and aligned by means of two threaded rods fixed to opposite ends of the PCB. This procedure was extremely laborious, requiring several days to complete the assembly of a single gate. Furthermore, the frames were expensive and the quality of the fabricated grids was inconsistent. Another technique is described in U.S. Pat. No. 6,664,545, where a surface with grooves thereon is employed for guiding a wire that is being wound around it to construct the BNG.
All of the above techniques are limited in how closely together the wires can be fixed in position, and hence also the resulting resolution that can be achieved. It is therefore desirable to provide improved BNG and other gates used in a BMD for modulating a beam of charged particles, and an improved method for making these gates.